This invention relates to printing and more particularly to high speed non-impact printing.
High speed non-impact printers are described in U.S. Pat. No. 3,550,153, issued to R. Haeberle et al. and assigned to the assignee of the present invention. Such printers include one or more print electrode pairs (comprising a field shaping electrode and a base electrode) configured about a donor sheet and a recipient sheet. On or near the surface of the donor sheet adjacent to the recipient sheet, thee is a relatively uniform reservoir of electrically conductive, mobile printing particles. The application of an electrical printing pulse signal across a print electrode pair, and the resultant pulsed electrical field, effects the transfer of mobile printing particles from regions on the surface of the donor sheet through which the electric field passes to adjacent regions on the surface of the recipient sheet.
Generally, each print electrode pair in such a printer is associated with an individual print signal generator which may be selectively activated to apply the printing pulse. The print signal generators for a printing means may have the form of one of many means known in the art for selectively generating high voltage pulse signals in response to a control signal. For example, a transistor switch may be used to drive a step-up transformer having its output connected across the print electrode pair. However, such print signal generators impose substantial limitations on the printing means since each print electrode pair requires an associated print signal generator including a transistor switch and a step-up transformer. A first such limitation is the substantial power dissipation and cost associated with the generation of a high voltage print pulse using the step-up transformer circuit configuration.
A second limitation is evident in printing means which utilize a multiple print electrode pair configuration, e.g., in an array of selectively operable cylindrical field shaping electrodes associated with a common base electrode. Due to the number of circuit elements and the physical size associated with each separate print signal generator (one for each electrode pair), a substantial limitation is imposed on the printing electrode size and spacing (and, as a result, printing resolution).
In addition, in such a multiple electrode printing means, the print signal generator associated with each print electrode pair generally provides printing pulses of a single voltage amplitude and consequent printing having a single optical density. Therefore, unless a substantially more complex (and correspondingly more expensive) pulse generator is provided for each printing electrode pair which is capable of providing controlled voltage amplitude printing pulses to selective printing electrode pairs, only two tone (e.g., black-white) printing may be achieved.